This invention relates to a heat transfer sheet, more particularly, to a heat transfer sheet capable of forming an image having excellent color reproducibility as well as sharpness of the printed image.
Various heat transfer methods have been known in the art, and among them, there has been proposed a method in which a sublimable dye is used as the recording material. The dye is carried on a substrate sheet such as a polyester film to form a heat transfer sheet. By using the transfer sheet, various full colors are formed on an image-receiving sheet having a dye receptive layer with a sublimable dye such as paper or plastic film.
In the above full-color heat transfer method, in most cases, by use of heat transfer sheets of 3 or 4 colors of yellow, magenta, cyan (and black) or a heat transfer sheet having these 3 or 4 colors formed plane successively on a continuous substrate sheet, a thermal head is actuated by the electrical signals formed by resolving the original into 3 or 4 colors to effect printing with matching of the 3 or 4 colors on one heat transfer image-receiving sheet, thereby reproducing the full-color image on the image-receiving sheet.
The image thus formed is very sharp, since the colorant used is a dye having excellent transparency, whereby the obtained image is excellent in reproducibility and gradation of the intermediate color, similar to the image according to the offset printing or gravure printing of the prior art, and further can form an image of high quality comparable with full-color photographic image.
In forming the color image comprising the 3 primary colors, all of the respective colors should have high spectral characteristics. In particular, in the case of a cyan dye, it should ideally have an absorption curve of so-called block type, having no absorption at around 540 nm or shorter and absorbing the wavelengths longer than that.
However, existing cyan dyes exhibit the so called hanging bell type absorption curve, with its skirt on the shorter wavelength side extending to the range of 540 nm or shorter, whereby there is the problem that the lightness and the chromaticity of the cyan image color formed becomes lower due to absorption of 540 nm or lower.
Such problem becomes more marked as the image density is made higher. Further, since the components of 540 nm or lower fall within the green light region, there is the problem that the intermediate color of green, etc. formed by primary color mixing with yellow dye, etc. during image formation becomes indistinct.
Also, generally speaking, dyes which are excellent in sharpness are inferior in light resistance and bleeding resistance. On the contrary, dyes which are excellent in light resistance and bleeding resistance have a tendency to be inferior in sharpness. Thus, both sharpness and other properties such as light resistance, etc. cannot be obtained easily in most cases. On the other hand, the image-receiving sheet comprises a resin layer having dye dyability formed on the surface of a film or a sheet in most cases. These resin layers, however, are generally tinted with yellow and therefore there is involved the problem that sharpness of the transferred dye is degraded.
As the method for solving the above drawbacks, it may be conceivable to add a fluorescent brightening agent in the dye receptive layer so as to extinguish the yellow tint of the receptive layer and also enhance sharpness of the transferred dye.
However, since fluorescent brightening agents are compounds which absorb UV-ray and emit blue to violet light, and inevitably tend to extinguish yellow color thereby causing a problem that color forming characteristics and sharpness of yellow colors are degraded, whereby there is the problem that color reproducibility and color forming characteristics of yellow colors of full-color originals are lowered.
Accordingly, an object of the present invention is to provide a heat transfer sheet capable of forming a transferred image having excellent sharpness and color reproducibility in a heat transfer recording system by use of a sublimable dye.